Conventional golf balls include multi-layer balls, which may have one or more wound layers. The difference in playing characteristics is significant among the three different types of balls: two-piece balls, wound balls and multi-layer balls. Two-piece balls are typically made with a single solid core encased by a cover material. These balls are generally most popular among recreational golfers, because they are durable and provide maximum distance. Typically, the solid core is made of polybutadiene chemically cross-linked with zinc diacrylate and/or similar cross-linking agents. The cover material comprises tough, cut-proof blends of one or more ionomers, such as SURLYN® sold commercially by DuPont or IOTEK® sold commercially by Exxon.
Wound balls typically have either a solid rubber or liquid-filled center, around which many yards of a stretched elastic thread or yarn are wound to form a core. The wound core is then covered with a durable ionomer cover, or a softer cover such as balata or polyurethane. Wound balls are generally softer than two-piece balls and can provide more spin, thus enabling skilled golfers to have more control over the ball's flight and placement.
Solid multi-layer golf balls may have one or more core layers, one or more intermediate layers and one or more cover layers. They are designed to overcome some of the undesirable features of conventional two-piece balls, such as hard feel, while maintaining the positive attributes of wound balls, such as increased initial velocity and distance. It is also desirable that multi-layer balls have similar “click and feel” and spin characteristics of wound balls.
Solid multi-layer golf balls can be produced using a variety of manufacturing techniques. For example, two or more cover layers may be molded around a conventional core with one or more intermediate layers interposed between the cover layers and the core. Alternatively, multi-layer balls may be formed from cores having more than one core layers and may optionally contain one or more intermediate and/or cover layers. Multi-layer balls may even comprise a conventional wound core around which at least one intermediate layer and/or at least one cover layer are formed. Typically, the outer layers of multi-layer golf balls are formed by molding them around the core or the preceding intermediate layer or cover layer. Conventional techniques for applying such layers include injection molding, compression molding and casting the layer material around the preceding core or layer.
It is desirable to obtain good adhesion between the various solid layers. If the adhesion between the layers is not acceptable, the performance or durability of the golf ball can be adversely affected. For example, poor adhesion can cause unattached areas to form between the layers that can result in separation of the layers when the ball is struck with a club. It is also known that the adhesion between the wound layer and the encasing layer can be improved when the encasing material flows into the voids among the windings during the molding process, resulting in improved adhesion.
The patent literature discloses a number of references teaching improved adhesion techniques. U.S. Pat. Nos. 6,103,166, 6,342,019, and 6,648,776 to Boehm et al., which are incorporated by reference in their entireties, relate to using textured surface profile that are integrally molded to the outer surface to improve adhesion between golf ball layers. The surface profile comprises peaks having heights between about 2 mils and about 15 mils.
U.S. Pat. No. 6,440,346 to Wai et al. relates to a golf ball comprising a core having outwardly extending projections, and an interstitial space layer of relatively less resilient material applied in the interstitial space between the projections on the surface of the core. A cover is then applied over the core and the interstitial space layer.
U.S. Pat. No. 6,605,243 to Masatani relates to a method of producing golf ball comprising a spherical elastic inner layer having outwardly extending projections with either round or polygonal shapes, and a thermoplastic resin outer layer covering the inner layer.
U.S. Pat. No. 6,761,846 to Murphy relates to a method of making a golf ball with an interior layer having outwardly extending protrusions to facilitate adhesion to the cover materials.
U.S. Pat. No. 5,837,183 to Inoue et al. relates to a method of molding golf ball comprising the steps of providing a thin film layer of a magnetic material on the inside surface of a spherical cavity of a mold, embedding an induction heating coil and a cooling channel beneath the magnetic material in the mold, generating a magnetic field via a high frequency oscillator to heat the cavity, introducing a predetermined amount of a molten stock material to fill the cavity, and feeding a coolant via the cooling channel to complete the molding process.
U.S. Pat. No. 6,585,607 to Tzivanis et al. relates to a process to increase adhesion between two adjacent layers in a golf ball comprising the steps of roughening the bonding surface of one layer, chlorinating the roughened surface, treating the surface of adjacent layer with a silicone-based adhesion promoter, and joining the layers.
However, there remains a need to improve interlayer adhesion in the process of making multi-layer golf balls.